284 COMPARATIVE ANATOMY. 



the former giving rise to the carotids and the latter forming the 

 large aortic root (Fig. 228, cc, ci, RA) ; an anastomosis remains 

 throughout life, however, between the fourth arch, which forms 

 the pulmonary artery, and the second and third (Fig. 228). This 

 is the ductus Botalli. 



The third arch varies greatly in its development ; it may be 

 present on one side only, or even may be entirely wanting. 



FIG. 228. ARTERIAL ARCHES or AN ADULT Salamandra maculosa, SHOWN SPREAD 

 OUT. (After J. E. V. Boas.) 



co, tr, comis (truncus) artcriosus ; 1 to 4, the four arterial arches ; ce, external caro- 

 tid ; cd, carotid gland ; ci, internal carotid ; the fourth arterial arch, which 

 gives rise to the pulmonary artery (Ap), has increased considerably in size rela- 

 tively, and is only connected by a delicate ductus Botalli (t) with the second and 

 third arches ; RA, root of the aorta ; ce, cesophageal vessels. 



In the larvaB of Anura there are also four branchial arteries 

 present on either side, but these are connected with the correspond- 

 ing veins by capillaries only, there being no direct anastomoses 

 (compare Fig. 227, a, a). The consequence of this is that all the 

 blood becomes oxygenated. 



In the adult Frog the third arterial arch becomes entirely 

 obliterated, and the first is completely separated from the second. 

 In other points the arrangement is similar to that seen in the 

 Salamander. 



Reptiles. As in all Amniota, the heart of Reptiles arises far 

 forwards in the neighbourhood of the gill-clefts, but on the forma- 

 tion of a neck it comes to lie much further back than is the case in 

 the Anamnia. 1 The vagus nerve, which supplies the heart, is 

 thus correspondingly elongated, and the carotid arteries and jugular 

 veins also become increased in length. 



The principal advance in structure as compared with the 



1 It lies furthest forwards in most Lizards and in Chelonians, while in Amphis- 

 kenians, Snakes, and Crocodiles it is situated much further back. 



